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Physics160Physics

160Lecture 1DC CircuitsR. JohnsonApril 2, 2012Room Change!•Starting with the 2ndlecture, on Wednesday, we willmeet in Room 231 of the ISB building (close to thePhiDttffi)PhysicsDepartmentoffice).April 2, 2012Physics 1602Course Organization•45% of the grade is on the lab work–20% on homework and 30% on the final exam–5% on classroomparticipation and reading

quizzesppgq•Be sure to make use of the course web page,including eCommons. Included areClttdtllb–Complete up-to-date syllabus–Course calendar–Class emailCltidt–Class notices andannouncements–Course handouts (I generally will not hand out hardcopy)•PSpicetutorialAClifliiit•ACanalysis of

generally available in my office in Nat Sci2, Room 323, incase you have urgent questions.–YouarewelcometousethetimeonFridaystofinishupyourYou

are

welcome

to

use

the

time

on

Fridays

to

finish

up

your

lab work or complete your homework assignment, includingPSpicesimulations.April 2, 2012Physics 1605Labs•This week you must do lab #1, DC circuits. The labwill be open today through Friday.Youneedtwoboundlabnotebookswithquadrule•You

need

two

bound

lab

notebooks

with

quad

rule

paper (the same as used in Physics 5 labs).–Each will be used on alternate weeks, to allow time forgradinggrading.•You need a kit of parts from me (provided by Prof.Kuttner in Room 176).•The lab writeup, done in the same notebook in whichyou take data, will be due by 2 pm the followingMonday.y–The writeupmust be neat enough to be easily read and mustdemonstrate that you did all measurements, clearly show theresults, and answer all of the questions in the manual.April 2, 2012Physics 1606–Graphs must be neat, by hand or by computer.The kitparts list:April 2, 2012Physics 1607Lecture 1 NotesVltfttildiff•Voltage: measure ofpotential

difference–Voltage “at” a node in a circuit generally means the potentialdifference between that point and “ground.”–Connect a voltmeter between two points to measure thepotential difference•Ideally the voltmeter should draw negligible current through theparallel path created.•Current (Amps): measure of the charge per unit timeflowingthroughadeviceorwire.flowing

througha

device

or

wire.–You have to break the circuit to insert the ammeter, so thatthe current flows through the ammeter.Ideallytheammeterwillhaveanegligiblevoltagedrop–Ideally

the

ammeter

will

have

a

negligible

voltage

drop.–You can also measure current by connecting a voltmeteracross a resistor in a circuit and making use of Ohm’s law:PPIVApril 2, 2012Physics 1608•Power:P=IVRVIMeasuring Current & VoltageMust have verylowresistance, soitdoesn’tdisturbit

the circuit.Kirchhoff’s loop ruleR1R2IV•The sum of potential changes around a closed loop is zero.•The is simply a special case of the rule for electrostaticfields:0dEAround any closed loop in space.0IRIRVVIApril 2, 2012Physics 16010021IRIRV21RRIKirchhoff’s Junction RuleI3R1R2I2I1213III213This is simply charge conservation for steady currents:•Charge cannot be created or disappear, andApril 2, 2012Physics 16011•Charge cannot pile up at the junction or any other point in the wire.Series and Parallel Combinations•Series: The equivalent resistance is greater than thatof the largest of the resistors.321eqRRRR•Parallel: the equivalent resistance is less than that ofthe smallest of the resistors.1111RRRR321敱RRRR=sumofthe“conductances”April 2, 2012Physics 16012

a series combination of a singlevoltage source and a singleresistor.resistor.•Measure or calculate the output voltage VABwith no load(open circuit).)iit(VV•Measure or calculate the output current with A shorted toB (short circuit).)circuitopen(ThVV)捩牣畩c潰敮(周VRApril 2, 2012Physics 16023)circuitshort (ThIRNorton Equivalent Circuit•Measure or calculate the output current IABwhen theoutputs are shorted together. This is INo.•MeasureorcalculatetheoutputvoltageVwhenthe•Measure

or

calculate

the

output

voltage

VABwhen

the

outputs are open.NABNoIVR•Note:NoINoThRRApril 2, 2012Physics 16024NoNoThRIVLab Precautions•Do not work alone in the lab.•Never open any chassis that uses A/C power while itispluggedinis

plugged

in.–In fact, don’t open any such chassis, period, without talkingto me or the TA first.Tthttthdidttibf•Turnthe metertothedesiredsettingbeforeconnecting it to the circuit.–Otherwise you are likely to blow a fuse or even damage thetmeter.•Measure the power supply voltage, and then turn itoff, beforeconnecting it to your circuit.•Do not make changes to your circuit wiring while it ispowered.•PoweroffallequipmentwhenfinishedApril 2, 2012Physics 16025Power

off

all

equipment

when

finished.More Lab Precautions•The return side of the oscilloscope input must always beconnected to ground.–Do not try to use the scope to measure the voltage across alidfhidcomponent unless one side ofthat componentis connectedtoground.•Do not connect a diode directly across a voltage source.Diillbblikldtit–Doing so willprobably quicklydestroyit.–Make sure that there is a resistor of at least 1 kohmin series withthe diode.Ttiittbtblit•Testyour circuitstage-by-stage as you assembleit.–Resist the urge to connect everything together all at once and thenturn the power on to see what happens.ThisisnotsoftwareAbuginyourcircuitcandestroycomponents–This

is

not

software.A

bug

in

your

circuit

can

destroy

components

and/or blow fuses!•No food or drink in the lab, period. This is a strict EH&SregulationthatfollowsfromtheoccasionaluseofradioactiveApril 2, 2012Physics 16026regulation

that

follows

from

the

occasional

use

of

radioactive

source in the room.Lab 1 Pointers•You may have to use a DVM for the current. The black analogVOM meters don’t seem to have a suitable range.–OrputtheVOMmeteronthe50A獣慬s慮a捯湮散ca㔶5佲灵p瑨t噏V浥瑥m潮瑨t㔰A獣慬s慮a捯湮散ca㔶5牥獩獴潲⁩渠灡牡汬敬⁦潲r愠〮㕭䄠獣慬攠潲‵afor a 5mA scale, etc.•(Calculated assuming a 5kresistance for the meter winding.)–Don’t connect the meter across a voltage source with it on thegammeter setting, or else the fuse willblow!•For the diode section, make measurements with the diode bothforward and backward.–You should be able to make a graph something like Figure 1.67 inthe textbook, except that the negative scale won’t go far enough toshow the breakdown.Alkdhfjtthitiidlttith•Also, make a secondgraphof

just

the positive side, plottingthelogarithm of the current on the vertical scale.–Be sure to use the 1N4148 (1N914 equivalent) diode (there are 4 inthe kit),not one of the zenerdiodes!April 2, 2012Physics 16027),•My eyes need a magnifying glass to read the labels.Resistor Color CodeApril 2, 2012Physics 16028